Copper(I) acetylide, or cuprous acetylide, is a chemical compound with the formula Cu2C2, known at least since 1856.[1] In the common form (a monohydrate with formula Cu
2C
2.H
2O) it is a reddish solid, that easily explodes when dry.

Copper acetylide can form inside pipes made of copper or an alloy with high copper content, which may result in violent explosion.[2] This was found to be the cause of explosions in acetylene plants, and led to abandonment of copper as a construction material in such plants.[3] Copper catalysts used in petrochemistry can also possess a degree of risk under certain conditions.[4]

Copper acetylide is the substrate of Glaser's reaction for the formation of polyynes. In a typical reaction, air is bubbled through a suspension of Cu
2C
2.H
2O in an amoniacal solution. The copper is oxidized to Cu2+ and forms a blue soluble complex with the ammonium species, leaving behind a black solid residue. The latter has been claimed to consist of carbyne, an elusive allotrope of carbon;[5] more precisely of "polyacetylyde" anions capped with residual copper(I) ions,

Freshly prepared copper acetylide is decomposed by hydrochloric acid into acetylene and copper(I) chloride. Samples that have been aged with exposure to air or to copper(II) ions liberate also higher polyynes H(−C≡C−)nH, with n from 2 to 6, when decomposed by hydrochloric acid. A "carbonaceous" residue of this decomposition also has the spectral signature of (−C≡C−)n chains. It has been conjectured that oxidation causes polymerization of the acetylide anions C2−
2 in the solid into carbyne-type anions .C(≡C−C≡)nC2− or polycumulene-type anions C(=C=C=)mC4−.[1]

Thermal decomposition of copper acetylide in vacuum is not explosive and leaves copper as a fine powder at the bottom of the flask, while depositing a fluffy very fine carbon powder on the walls. On the basis of spectral data, this powder was claimed to be carbyne C(−C≡C−)nC rather than graphite as expected.[1]

Though not practically useful as an explosive due to high sensitivity and reactivity towards water[citation needed], it is interesting as a curiosity because it is one of the very few explosives that do not liberate any gaseous products upon detonation.

The formation of copper acetylide when a gas is passed through a solution of copper(I) chloride is used as a test for the presence of acetylene.

The reaction of Cu+ with alkyne occurs only if terminal hydrogen is present (as it is slight acidic in nature). Thus this reaction is used for identifification of terminal alkynes.